Degradation detection method for an engine having a NOx sensor

ABSTRACT

An engine diagnostic system for use with an internal combustion engine coupled to a lean NOx trap with a NOx sensor coupled downstream of the trap. Degradation of the NOx sensor is determined by estimated NOx exiting the trap based on engine out NOx and various other operating conditions. When the difference between the measured NOx and estimated NOx is greater than a predetermined value for a predetermined duration, degradation is indicated.

[0001] The present application claims priority from provisionalapplication U.S. Serial No. 60/190,249, filed Mar. 17, 2000.

FIELD OF THE INVENTION

[0002] The invention relates to a system and method for determiningdegradation of a NOx sensor coupled to an internal combustion engine.

BACKGROUND OF THE INVENTION

[0003] Engine and vehicle fuel efficiency can be improved by lean burninternal combustion engines. To reduce emissions, these lean burnengines are coupled to emission control devices known as three-waycatalytic converters optimized to reduce CO, HC, and NOx. When operatingat air-fuel ratio mixtures lean of stoichiometry, an additionalthree-way catalyst, known as a NOx trap or catalyst, is typicallycoupled downstream of the three-way catalytic converter, where the NOxtrap is optimized to further reduce NOx. The NOx trap typically storesNOx when the engine operates lean-and release NOx to be reduced when theengine operates rich or near stoichiometry.

[0004] One method to provide emission control in a lean burn engine usesa sensor downstream of the NOx trap. The sensor is capable of measuringan amount of NOx in exhaust gas exiting the NOx trap. Engine air-fuelratio is changed from lean of stoichiometry to rich of stoichiometrywhen measured NOx reaches a predetermined threshold. Such a method isdescribed in U.S. Pat. No. 5,942,199.

[0005] The inventors herein have recognized a disadvantage with theabove approach. In particular, if the output of the NOx sensorinadvertently indicates a NOx concentration greater than thepredetermined threshold when NOx concentration is actually less that thethreshold, lean operation will be ended prematurely.

[0006] In other words, lean operation will be ended when it is actuallypossible to continue lean operation.

SUMMARY OF THE INVENTION

[0007] An object of the invention claimed herein is to provide a methodfor determining degradation of a sensor coupled to an internalcombustion engine. The above object is achieved, and disadvantages ofprior approaches overcome, by a method for determining degradation in anemission control system coupled to an internal combustion engine havingan emission control device coupled downstream of the engine, the methodcomprising: providing an output signal of a sensor coupled downstream ofthe emission control device, said output signal indicative of an exhaustgas constituent flowing through the emission control system; generatingan estimate of said exhaust gas constituent based at least on an engineoperating condition; and indicating when said estimate of said exhaustgas constituent and said second quantity differ by a predeterminedvalue.

[0008] By comparing an actual sensor output to an estimate based onother operating conditions, it is possible to determine when the sensoroutput has degraded.

[0009] An advantage of the above aspect of the present invention isimproved emissions is achieved since it is possible to determine whenthe sensor has degraded.

[0010] Other objects, features and advantages of the present inventionwill be readily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The objects and advantages described herein will be more fullyunderstood by reading an example of an embodiment in which the inventionis used to advantage, referred to herein as the Description of PreferredEmbodiment, with reference to the drawings, wherein:

[0012]FIGS. 1-2 are block diagrams of an embodiment wherein theinvention is used to advantage; and

[0013]FIGS. 3-15 are high level flow charts of various operationsperformed by a portion of the embodiment shown in FIG. 1.

DESCRIPTION OF THE INVENTION

[0014] Direct injection spark ignited internal combustion engine 10,comprising a plurality of combustion chambers, is controlled byelectronic engine controller 12 as shown in FIG. 1. Combustion chamber30 of engine 10 includes combustion chamber walls 32 with piston 36positioned therein and connected to crankshaft 40. In this particularexample, piston 30 includes a recess or bowl (not shown) to help informing stratified charges of air and fuel. Combustion chamber 30 isshown communicating with intake manifold 44 and exhaust manifold 48 viarespective intake valves 52 a and 52 b (not shown), and exhaust valves54 a and 54 b (not shown). Fuel injector 66 is shown directly coupled tocombustion chamber 30 for delivering liquid fuel directly therein inproportion to the pulse width of signal fpw received from controller 12via conventional electronic driver 68. Fuel is delivered to fuelinjector 66 by a conventional high pressure fuel system (not shown)including a fuel tank, fuel pumps, and a fuel rail.

[0015] Intake manifold 44 is shown communicating with throttle body 58via throttle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC), which isalso utilized during idle speed control. In an alternative embodiment(not shown), which is well known to those skilled in the art, a bypassair passageway is arranged in parallel with throttle plate 62 to controlinducted airflow during idle speed control via a throttle control valvepositioned within the air passageway.

[0016] Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold48 upstream of catalytic converter 70. In this particular example,sensor 76 provides signal UEGO to controller 12, which converts signalUEGO into a relative air-fuel ratio λ. Signal UEGO is used to advantageduring feedback air-fuel ratio control in a manner to maintain averageair-fuel ratio at a desired air-fuel ratio as described later herein. Inan alternative embodiment, sensor 76 can provide signal EGO (not shown),which indicates whether exhaust air-fuel ratio is either lean ofstoichiometry or rich of stoichiometry.

[0017] Conventional distributorless ignition system 88 provides ignitionspark to combustion chamber 30 via spark plug 92 in response to sparkadvance signal SA from controller 12.

[0018] Controller 12 causes combustion chamber 30 to operate in either ahomogeneous air-fuel ratio mode or a stratified air-fuel ratio mode bycontrolling injection timing. In the stratified mode, controller 12activates fuel injector 66 during the engine compression stroke so thatfuel is sprayed directly into the bowl of piston 36. Stratified air-fuelratio layers are thereby formed. The strata closest to the spark plugcontains a stoichiometric mixture or a mixture slightly rich ofstoichiometry, and subsequent strata contain progressively leanermixtures. During the homogeneous mode, controller 12 activates fuelinjector 66 during the intake stroke so that a substantially homogeneousair-fuel ratio mixture is formed when ignition power is supplied tospark plug 92 by ignition system 88. Controller 12 controls the amountof fuel delivered by fuel injector 66 so that the homogeneous air-fuelratio mixture in chamber 30 can be selected to be substantially at (ornear) stoichiometry, a value rich of stoichiometry, or a value lean ofstoichiometry. Operation substantially at (or near) stoichiometry refersto conventional closed loop oscillatory control about stoichiometry. Thestratified air-fuel ratio mixture will always be at a value lean ofstoichiometry, the exact air-fuel ratio being a function of the amountof fuel delivered to combustion chamber 30. An additional split mode ofoperation wherein additional fuel is injected during the exhaust strokewhile operating in the stratified mode is available. An additional splitmode of operation wherein additional fuel is injected during the intakestroke while operating in the stratified mode is also available, where acombined homogeneous and split mode is available.

[0019] Nitrogen oxide (NOx) absorbent or trap 72 is shown positioneddownstream of catalytic converter 70. NOx trap 72 absorbs NOx whenengine 10 is operating lean of stoichiometry. The absorbed NOx issubsequently reacted with HC and catalyzed during a NOx purge cycle whencontroller 12 causes engine 10 to operate in either a rich mode or anear stoichiometric mode.

[0020] Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, anelectronic storage medium for executable programs and calibrationvalues, shown as read-only memory chip 106 in this particular example,random access memory 108, keep alive memory 110, and a conventional databus.

[0021] Controller 12 is shown receiving various signals from sensorscoupled to engine 10, in addition to those signals previously discussed,including: measurement of inducted mass air flow (MAF) from mass airflow sensor 100 coupled to throttle body 58; engine coolant temperature(ECT) from temperature sensor 112 coupled to cooling sleeve 114; aprofile ignition pickup signal (PIP) from Hall effect sensor 118 coupledto crankshaft 40 giving an indication of engine speed (RPM); throttleposition TP from throttle position sensor 120; and absolute ManifoldPressure Signal MAP from sensor 122. Engine speed signal RPM isgenerated by controller 12 from signal PIP in a conventional manner andmanifold pressure signal MAP provides an indication of engine load.

[0022] In this particular example, temperature Tcat of catalyticconverter 70 and temperature Ttrp of NOx trap 72 are inferred fromengine operation as disclosed in U.S. Pat. No. 5,414,994, thespecification of which is incorporated herein by reference. In analternate embodiment, temperature Tcat is provided by temperature sensor124 and temperature Ttrp is provided by temperature sensor 126.

[0023] Fuel system 130 is coupled to intake manifold 44 via tube 132.Fuel vapors (not shown) generated in fuel system 130 pass through tube132 and are controlled via purge valve 134. Purge valve 134 receivescontrol signal PRG from controller 12.

[0024] Exhaust sensor 140 is a sensor that produces two output signals.First output signal (SIGNAL1) and second output signal (SIGNAL2) areboth received by controller 12. Exhaust sensor 140 can be a sensor knownto those skilled in the art that is capable of indicating both exhaustair-fuel ratio and nitrogen oxide concentration.

[0025] In a preferred embodiment, SIGNAL1 indicates exhaust air-fuelratio and SIGNAL2 indicates nitrogen oxide concentration. In thisembodiment, sensor 140 has a first chamber (not shown) in which exhaustgas first enters where a measurement of oxygen partial pressure isgenerated from a first pumping current. Also, in the first chamber,oxygen partial pressure of the exhaust gas is controlled to apredetermined level. Exhaust air-fuel ratio can then be indicated basedon this first pumping current. Next, the exhaust gas enters a secondchamber (not shown) where NOx is decomposed and measured by a secondpumping current using the predetermined level. Nitrogen oxideconcentration can then be indicated based on this second pumpingcurrent.

[0026] In one aspect of the present invention, a determination ofdegradation of the nitrogen oxide concentration measurement can be madeif it is determined that the exhaust air-fuel ratio measurement isdegraded. This is because nitrogen oxide concentration is not accuratelydetected in the second chamber unless the first chamber controls oxygenpartial-pressure properly. In other words, if it is determined thatoperation of the first chamber (where partial pressure of oxygen ismeasured) is degraded, then it is possible to determine that the secondsignal (SIGNAL2) indicating nitrogen oxide concentration is degraded asdescribed later herein with particular reference to FIG. 13.

[0027] Referring now to FIG. 2, a port fuel injection engine 11 is shownwhere fuel is injected through injector 66 into intake manifold 44.Engine 11 is operated homogeneously substantially at stoichiometry, richof stoichiometry, or lean of stoichiometry. Fuel is delivered to fuelinjector 66 by a conventional fuel system (not shown) including a fueltank, fuel pumps, and a fuel rail.

[0028] Those skilled in the art will recognize, in view of thisdisclosure, that the methods of the present invention can be used toadvantage with either port fuel injected or directly injected engines.

[0029] Referring now to FIGS. 3-5, routines for determining performanceimpacts of operating in various engine operating conditions aredescribed. In a preferred embodiment, performance impact is a fueleconomy percentage impact over stoichiometric operation. The impact canbe a benefit, where fuel is saved over stoichiometric operation, or afuel loss. In other words, the following routines determine fuel economysaved relative to stoichiometric operation or fuel economy lost relativeto stoichiometric operation. However, those skilled in the art willrecognize in view of this disclosure various other performance impactsthat can-be used to compare different operation modes such as, forexample, fuel usage impact, fuel efficiency impact, fuel savings, fuelloss, engine efficiency impact, fuel savings per distance traveled bythe vehicle, or a drivability impact.

[0030] Referring now specifically to FIG. 3, a routine is described fordetermining a maximum fuel economy benefit that can be provided whenoperating lean, assuming that emission control device 72 has beendecontaminated. More specifically, in a preferred embodiment, that asulfur decontamination has been completed. In other words, the routinedetermines the maximum potential fuel economy benefit that can beprovided after performing a decontamination cycle. First, in step 308,counter j is reset equal to zero. Next, in step 310, a determination ismade as to whether a decontamination cycle has just been completed. Adecontamination cycle, as described herein, refers to any operatingcycle where engine operating conditions are changed to remove acontaminant. For example, a sulfur decontamination cycle where exhaustgas temperature is raised and the engine is operated substantially at orrich of stoichiometry to remove sulfur contaminating emission controldevice 72 is one such decontamination cycle. When the answer to step 310is YES, the routine continues to step 312 where parameter OLD_FE_MAX isset equal to parameter FILTERED_FE_MAX. Also, in step 312, counter j isset equal to one. Counter j keeps track of the number of NOx fill/purgecycles after a decontamination cycle over which the maximum fuel economybenefit is average. Next, in step 314, a determination is made as towhether a NOx fill/purge cycle has just been completed.. When the answerto step 314 is YES, a determination is made as to whether counter jequals one. When the answer to step 316 is YES, the routine continues tostep 318. In step 318, the routine calculates temporary fuel economybenefit (FE_TEMP_(j)) based on current fuel economy benefit (FE_CUR),where current fuel economy benefit is calculated as described below.This temporary fuel economy benefit represents the fuel economy benefitaveraged over a NOx fill/purge cycle that is achieved compared tooperating the engine substantially at stoichiometry. Next, in step 320,maximum fuel economy benefit (FE_MAX) is calculated based on temporaryfuel economy benefit. Next, in step 322, counter j is incremented. Next,in step 324, a determination is made as to whether counter j is greaterthan predetermined value J1. Predetermined value J1 represents thenumber of NOx fill/purge cycles after a decontamination cycle over whichmaximum fuel economy benefit, provided by lean operation relative tostoichiometric operation, is calculated. In a preferred embodiment,predetermined value J1 represents the number of NOx fill/purge cyclesafter a decontamination cycle over which maximum fuel economy benefit isaveraged. This averaging allows variations in vehicle operatingconditions to be accounted in determining maximum fuel economy benefitso that a representative value is obtained. When the answer to step 324is YES, the filtered maximum fuel economy benefit (FIL_FE_MAX) is setequal to maximum fuel economy benefit.

[0031] Continuing with FIG. 3, when the answer to step 316 is NO,temporary fuel economy benefit (FE_TEMP_(j)) is calculated in step 326based on current fuel economy benefit (FE_CUR). Current fuel economybenefit (FE_CUR) represents the current fuel economy benefit relative tostoichiometric operation provided by lean operation and is calculatedbased on operating conditions. In particular, as described in U.S.patent application Ser. No. 09/528,217, titled “METHOD AND APPARATUS FORCONTROLLING LEAN-BURN ENGINE BASED UPON PREDICTED PERFORMANCE IMPACT”,filed concurrently with the present application on Mar. 17, 2000,attorney docket number 199-0979, assigned to the same assignee as thepresent application, which is hereby expressly incorporated byreference, a performance impact is set as a percentage fuel economybenefit/loss associated with engine operation at a selected lean or richoperating condition relative to a reference stoichiometric operatingcondition at MBT, the controller 12 first determines whether thelean-burn feature is enabled. If the lean-burn feature is enabled as,for example, indicated by the lean-burn running flag LB_RUNNING_FLGbeing equal to logical one, the controller 12 determines a first valueTQ_LB representing an indicated torque output for the engine whenoperating at the selected lean or rich operating condition, based on itsselected air-fuel ratio LAMBSE and the degrees DELTA_SPARK of retardfrom MBT of its selected ignition timing, and further normalized forfuel flow. Then, controller 12 determines a second value TQ_STOICHrepresenting an indicated torque output for the engine 10 when operatingwith a stoichiometric air-fuel ratio at MBT, likewise normalized forfuel flow. In particular, TQ_LB is determined as a function of desiredengine torque, engine speed, desired-air-fuel ratio, and DELTA_SPARK.Further, TQ_STOICH is determined as a function of desired engine torqueand engine speed. Next, the controller 12 calculates the lean-burntorque ratio TR_LB by dividing the first normalized torque value TQ_LBwith the second normalized torque value TQ_STOICH.

[0032] Continuing, the controller 12 determines a value SAVINGSrepresentative of the cumulative fuel savings to be achieved byoperating at the selected lean operating condition relative to thereference stoichiometric operating condition, based upon the air massvalue AM, the current (lean or rich) lean-burn air-fuel ratio (LAMBSE)and the determined lean-burn torque ratio TR_LB, wherein

SAVINGS=SAVINGS+(AM*LAMBSE*14.65*(1−TR)).

[0033] The controller 12 then determines a value DIST_ACT_CURrepresentative of the actual miles traveled by the vehicle since thestart of the last trap purge or desulfurization event. While the“current” actual distance value DIST_ACT_CUR is determined in anysuitable manner, in the exemplary system, the controller 14 determinesthe current actual distance value DIST ACT_CUR by accumulating detectedor determined instantaneous values VS for vehicle speed.

[0034] Because the fuel economy benefit to be obtained using thelean-burn feature is reduced by the “fuel penalty” of any associatedtrap purge event, in the exemplary system, the controller 12 determinesthe “current” value FE_CUR for fuel economy benefit only once per NOxfill cycle. And, because the purge event's fuel penalty is directlyrelated to the preceding trap “fill,” the current fuel economy benefitvalue FE_CUR is preferably determined at the moment that the purge eventis deemed to have just been completed, as described below.

[0035] Continuing with FIG. 3, in step 328, maximum fuel economy benefitis calculated as a function (f₁) of maximum fuel economy benefit andtemporary fuel economy benefit. In this way, the fuel economy benefitprovided by a decontaminated emission control device is filtered overseveral NOx fill/purge cycles. In a preferred embodiment, the filteringis performed by a rolling average function of the form in the followingequation where (fk) is a filter coefficient between zero and one. Thoseskilled in the art will recognize, in view of this disclosure, this as asingle pole low pass filter.

output=(1−fk)output+(fk)input or,

output=(1−fk)old_output+(fk)input, old_output=output

[0036] Thus, according to the present invention, it is possible todetermine the fuel economy benefit provided by a decontaminated emissioncontrol device.

[0037] Referring now to FIG. 4, a routine is described for determiningthe present, or current, fuel economy benefit that is being provided byoperating lean of stoichiometry with the emission control device 72 inits present state, be it contaminated or decontaminated. First, in step410, a determination is made as to whether a NOx fill/purge cycle hasjust been completed. When the answer to step 410 is YES, the routinecontinues to step 412 where parameter OLD_FE_CUR is set equal toparameter FIL_FE_CUR. Next, in step 414, the routine calculates thecurrent fuel economy benefit (FE_CUR). Next, in step 416, the routinecalculates the filtered current fuel economy benefit (FIL_FE_CUR) basedon a filtered value of the current fuel economy benefit, and parameterOLD_FE_CUR. In other words, the current fuel economy benefit(FIL_FE_CUR) represents the fuel economy benefit that will be realizedif the system continues to operate as it currently does and nodecontamination is performed. Accordingly, (FIL_FE_CUR) is the fueleconomy benefit that will be achieved by not performing adecontamination cycle.

[0038] In a preferred embodiment, function (f₂) represents the rollingaverage function described above herein. Thus, according to the presentinvention, a fuel economy benefit averaged over several NOx fill/purgecycles can be determined. This value can then be used to advantage invarious ways, since it indicates an on-line measure of the improved fueleconomy performance provided by lean operation averaged to removecycle-to-cycle variation.

[0039] Referring now to FIG. 5, a routine is described for determining afuel economy penalty experienced by performing a decontamination cycle.More specifically, in a preferred embodiment, a decontamination cyclethat removes NOx. First, in step 510, a determination is made as towhether a decontamination cycle has just been completed. When the answerto step 510 is YES, the routine continues to step 512 where a fueleconomy penalty is calculated. The current fuel economy penalty of thelast decontamination cycle (CUR_FE_PENALTY) is calculated by dividingthe excess fuel used to generate heat or the excess fuel used to operatein one condition compared to another condition by the distance betweendecontamination cycles. In other words, the penalty of performing adecontamination cycle is spread over the distance between twodecontamination cycles. Next, in step 514, a filtered fuel economypenalty is calculated by filtering the current fuel economy penaltyaccording to function (f₃) which, in a preferred embodiment, representsthe rolling average function describe above herein. Thus, according tothe present invention, it is possible to determine the fuel economypenalty experienced by performing a decontamination cycle. In analternative embodiment, the fuel economy penalty to perform adecontamination cycle can be set to a predetermined value.

[0040] Those skilled in the art will recognize, in view of thisdisclosure, various alterations of the present invention that achieve asimilar result. For example, the average excess fuel used during severaldecontamination cycles can be divided by the total distance between allof the decontamination cycles, thereby providing an averaged fueleconomy penalty for performing a decontamination cycle.

[0041] In an alternate embodiment, fuel economy penalty to perform adecontamination cycle can be stored as a function of vehicle and/orengine operating parameters. For example, fuel economy penalty can bestored versus vehicle speed and exhaust gas temperature experiencedbefore performing said decontamination cycle. Those skilled in the artwill recognize, in view of this disclosure, various other factors thataffect a fuel economy penalty to perform a decontamination cycle suchas, for example, engine speed, mass air flow, manifold pressure,ignition timing, air-fuel ratio, exhaust gas recirculation amount, andengine torque.

[0042] In yet another embodiment, fuel economy penalty can be determinedas now described. First, controller 12 updates a stored valueDIST_ACT_DSX representing the actual distance that the vehicle hastraveled since the termination or “end” of the immediately precedingdesulfurization, or decontamination, event. Then, the controller 12determines whether a desulfurization event is currently in progress.While any suitable method is used for desulfurizing the trap, anexemplary desulfurization event is characterized by operation of some ofthe engine's cylinders with a lean air-fuel mixture and other of theengine's cylinders with a rich air-fuel mixture, thereby generatingexhaust gas with a slightly-rich bias. Next, the controller 12determines the corresponding fuel-normalized torque values TQ_DSX_LEANand TQ_DSX_RICH, as a function of current operating conditions. Inparticular, TQ_DSX_LEAN and TQ_DSX_RICH are determined as functions ofdesired engine torque, engine speed, desired air-fuel ratio, andDELTA_SPARK. Then, the controller 12 further determines thecorresponding fuel-normalized stoichiometric torque value TQ_STOICH as afunction of desired engine torque and engine speed. The controller 12then calculates a cumulative fuel economy penalty value, as follows:

PENALTY=PENALTY+(AM/2*LAMBSE*14.65*(1-TR _(—)DSX_LEAN))+(AM/2*LAMBSE*14.65*(1-TR _(—) DSX_RICH))

[0043] Then, the controller 12 sets a fuel economy penalty calculationflag to thereby ensure that the current desulfurization fuel economypenalty measure FE_PENALTY_CUR is determined immediately upontermination of the on-going desulfurization event.

[0044] If the controller 12 determines that a desulfurization event hasjust been terminated, the controller 12 then determines the currentvalue FE_PENALTY_CUR for the fuel economy penalty associated with theterminated desulfurization event, calculated as the cumulative fueleconomy penalty value PENALTY divided by the actual distance valueDIST_ACT_DSX. In this way, the fuel economy penalty associated with adesulfurization event is spread over the actual distance that thevehicle has traveled since the immediately-prior desulfurizatidn event.Next, the controller 12 calculates a rolling average value FE_PENALTY ofthe last m current fuel economy penalty values FE_PENALTY_CUR to therebyprovide a relatively-noise-insensitive measure of the fuel economyperformance impact of such desulfurization events. The value FE_PENALTYcan be used in place of value FIL_FE_PENALTY. By way of example only,the average negative performance impact or “penalty” of desulfurizationtypically ranges between about 0.3 percent to about 0.5 percent of theperformance gain achieved through lean-burn operation. Finally, thecontroller 23 resets the fuel economy penalty calculation flagFE_PNLTY_CALC_FLG, along with-the previously determined (and summed)actual distance value DIST_ACT_DSX and the current fuel economy penaltyvalue PENALTY, in anticipation for the next desulfurization event.

[0045] Referring now to FIG. 6, a routine is described for determiningwhether to commence, or begin, a decontamination cycle. First, in step610, a determination is made as to whether the maximum potential fueleconomy benefit provided a decontaminated emission control device minusthe current fuel economy benefit being provided by the decontaminationcycle in its present condition is greater than the fuel economy penaltyexperienced by performing a decontamination cycle. In particular, thedifference between parameter FIL_FE_MAX and parameter FIL_FE_CUR iscompared to parameter FIL_FE_PENALTY. When the answer to step 610 isYES, the routine has determined that greater fuel economy can beprovided by performing a decontamination cycle rather than continuingwith operating the engine lean of stoichiometry and performing NOxfill/purge cycles. When the answer to step 610 is NO, the routine hasdetermined that greater fuel economy can be provided by continuingoperation in the present condition. In other words, operating with theemission control device in its present condition provides better fueleconomy than attempting to improve operation of the emission controldevice by performing a decontamination cycle. Next, in step 612, adetermination is made as to whether normalized NOx storage ability(FIL_NOX_STORED) of the emission control device is less than limit valueC1. Normalized stored NOx (FIL_NOX_STORED) is calculated as describedlater herein with particular reference to FIGS. 9 and 10. When theanswer to step 612 is YES, the routine continues to step 613 where adetermination is made as to whether vehicle distance traveled since thelast decontamination cycle is greater than limit distance(DISTANCE_LIMIT). When the answer to step 613 is YES, the routinecontinues to step 614 where a determination is made as to whetherparameter Al is equal to one. Parameter A1 is determined based onvehicle activity as described later herein with particular reference toFIG. 7. When the answer to step 614 is YES a decontamination cycle isbegun in step 616. The embodiment shown in FIG. 6 is that for theexample of a port fuel injected engine. In an alternate embodiment,which can be used for direct injection engines, step 614 is eliminated.This is because in port fuel injected engines, it is challenging toprovide well controlled decontamination temperatures under all operatingconditions. However, in a direct injection engine, since fuel can beinjected during the exhaust stroke to heat the exhaust system,decontamination can be performed at almost any time.

[0046] Referring now to FIG. 7, a routine is described for determiningvehicle activity. First, in step 710, a routine calculates engine power(Pe). In a preferred embodiment, this is the actual engine power;however, in a preferred embodiment, desired engine power can be used.Also, various other parameters can be used in place of engine power suchas, for example, vehicle speed, engine speed, engine torque, wheeltorque, or wheel power. Next, in step 712, engine power (Pe) is filteredwith a high pass filter G₁(s), where s is the Laplace operator, known tothose skilled in the art, to produce high pass filtered engine power(HPe). Next, in step 714, the absolute value (AHPe) of the pass filteredengine power (HPe) is calculated. In step 716, the absolute value (AHPe)is low pass filtered with filter G₁(s) to produce signal (LAHPe). Instep 718, adjustment factor K1 calculated as a predetermined function gof signal (LAHPe). Then, in step 720, a determination is made as towhether signal (LAHPe) is less than the calibration parameter(DESOX_VS_ACT_ENABLE_CAL). When the answer to step 720 is YES, parameterA1 is set to one in step 722. Otherwise, value A1 is set to zero in step724.

[0047] Referring now to FIG. 8, a graph of function g shows howadjustment factor K1 varies as a function of signal (LAHPe) in apreferred embodiment. As shown in the preferred embodiment, as vehicleactivity increases, adjustment factor K1 is reduced. As vehicle activitydecreases, adjustment factor K1 is increased to a maximum value of 0.7.

[0048] Referring now to FIGS. 9 and 10, a routine for determining NOxstored in an emission control device is described. In particular, theroutine describes a method for determining a consistent measure of NOxstored that can be averaged over several NOx purge/fill cycles. First,in step 910, a determination is made as to whether a NOx purge has justbeen completed. In an alternate embodiment, an additional check as towhether lean operation has commenced can also be used. When the answerto step 910 is YES, NOx stored estimated (NOX_STORED) is reset to zeroin step 912. In particular, the routine assumes that a complete NOxpurge was completed and all stored NOx was removed. However, in analternate embodiment, if only part of the NOx was purged, NOx stored instep 912 would be set to this partial value rather than zero. Next, instep 913, flag Z is set to zero to indicate that the stored NOx value isnot fully estimated. Next, in step 914, a determination is made as towhether the engine is operating lean of stoichiometry. When the answerto step 914 is YES, the routine continues to step 916. In step 916, acalculation of feedgas NOx (NOX_FG) based on operating conditions isgenerated. In particular, feedgas NOx generated by the engine iscalculated based on function (h1) using operating conditions such as,for example, SIGNAL1 (or desired air-fuel ratio of the engine), mass airflow (mair), engine temperature (TENG), and engine speed (RPM). Thisfeedgas NOx can then be used to represent the NOx entering NOx trap 72.Those skilled in the art will recognize in view of this disclosure thatvarious additional factors can be used such as factors that account foran NOx storage or reduction due to activity of three-way catalyst 70.

[0049] Continuing with FIG. 9, in step 918, a determination is made asto whether the ratio of NOx exiting trap 72 to NOx entering trap 72 isgreater than threshold C2. For example, threshold C2 can be set to 0.65.When the answer to step 918 is NO, a NOx difference (NOX_DELTA) iscalculated between NOx entering (NOX_FG) and NOx exiting (SIGNAL2) instep 920. Next, in step 922, an accumulated NOx storage (NOX_STORED) isdetermined by numerically summing NOx difference (NbX_DELTA). When theanswer to step 918 is YES, flag Z is set to one to indicate that aconsistent measure of NOx stored has been completed and fully estimated.

[0050] Referring now to FIG. 10, in step 1012, a determination is madeas to whether a NOx purge has just been completed. When the answer tostep 1012 is YES, the routine continues to step 1014. In step 1014,filtered normalized NOx stored (FIL_NOX_STORED) is calculated byfiltering NOx stored (NOX_STORED) according to function (f₄) which, in apreferred embodiment, represents the rolling average function describeabove herein.

[0051] Thus, according to the present invention, it is possible tocalculate a value representing a consistent and normalized NOx storagevalue that can be used in determining degradation and determiningwhether to perform a decontamination cycle.

[0052] Referring now to FIG. 11, a routine is described for using firstoutput signal (SIGNAL1) of sensor 140 for performing closed loopair-fuel ratio control. First, in step 1110, a determination is made asto whether the absolute value of the difference between SIGNAL1 andstoichiometric air-fuel ratio (air_fuel_stoich) is greater than apredetermined difference (D1). In other words, a determination is madeas to whether the first output signal of exhaust sensor 140 isindicating an exhaust air-fuel ratio away from stoichiometry. When theanswer to step 1110 is YES, the routine continues to step 1112. In step1112, the routine determines an air-fuel error (afe) based on thedifference between desired air-fuel ratio (air_fuel_desired) and thefirst output signal (SIGNAL1). Next, in step 1114, the routine generatesfuel injection signal (fpw) based on the determined error (afe) and thecylinder charge (m_cyl_air) and desired air-fuel ratio(air_fuel_desired). In addition, function g2 is used to modify theair-fuel error (afe) and can represent various control functions suchas, for example, a proportional, integral and derivative controller.Also, function g1 is used to convert the desired mass of fuel enteringthe cylinder into a signal that can be sent to fuel injector 66. Also,those skilled in the art will recognize, in view of this disclosure,that various other corrections involving information from other exhaustgas sensors can be used. For example, additional corrections from sensor76 can be used.

[0053] When the step 1110 is NO, the routine continues to step 1116 andcalculates fuel injection signal (fpw) based on the cylinder chargeamount and the desired air-fuel ratio using function g1. Thus, accordingto the present invention, it is possible to improve open-loop fuelingcontrol using the first output of sensor 140, which is locateddownstream of NOx trap 72, whenever the first output signal indicates avalue away from stoichiometry. In this way, NOx storage and oxygenstorage, as well as NOx reduction, do not adversely closed-loop feedbackair-fuel control using a sensor located downstream of a NOx trap.

[0054] Referring now to FIG. 12, an alternate routine to that describedin FIG. 11 is shown. In this alternate routine, various timers are usedto gate out the first output of exhaust sensor 140 for use in feedbackair-fuel ratio control whenever it is determined that one of thefollowing conditions is present: oxygen is being stored in NOx trap 72;and/or nitrogen oxide is being released and reduced by a reducingconstituent in the exhaust gas in NOx trap 72. Also, this alternateembodiment can be used to advantage to determine when to enablemonitoring of exhaust sensor 140, as described later herein withparticular reference to FIGS. 13 and 14.

[0055] Continuing with FIG. 12, in step 1210, a determination is made asto whether the desired air-fuel ratio (air_fuel_desired) has beenchanged. In particular, a determination is made as to whether thedesired air-fuel ratio has changed from rich or stoichiometric to lean,or whether the desired air-fuel ratio has changed from lean tostoichiometric or rich. When the answer to step 1210 is YES, the counterC3 is reset to zero in step 1212. Otherwise, in step 1214, counter C3 isincremented. Next, in step 1216, a determination is made as to whetherthe desired air-fuel ratio is stoichiometric or rich. When the answer tostep 1216 is YES, a determination is made as to whether counter C3 isgreater than threshold value D2 in step 1218. Otherwise, when the answerto step 1216 is NO, a determination is made as to whether counter C3 isgreater than threshold value D3 in step 1220. When the answer to eitherstep 1218 or step 1220 is YES, the routine enables monitoring in step1222.

[0056] In other words, duration D2 and duration D3 represent periodsbefore which first output of exhaust sensor 140 cannot be used forfeedback control because it will indicate stoichiometric even when theexhaust air-fuel ratio entering NOx trap 72 is not stoichiometric. Thus,when changing from stoichiometric or rich to lean, first output ofexhaust sensor 140 is valid for monitoring or feedback control afterduration D3. Similarly, when changing from lean operation to rich orstoichiometric operation, first output of exhaust sensor 140 is validfor monitoring or feedback control after duration D2. In a preferredembodiment, duration D2 is based on oxygen storage of trap 72 andduration D3 is based on both oxygen storage and NOx storage of trap 72.Stated another way, once the oxygen storage is saturated when changingfrom rich to lean, SIGNAL1 is indicative of the air-fuel ratio enteringtrap 72. And, once the oxygen stored and NOx stored is reduced whenchanging from lean to rich, SIGNAL1 is indicative of the air-fuel ratioentering trap 72.

[0057] Continuing with FIG. 12, in step 1224, a determination ofair-fuel error (afe) is made by subtracting desired air-fuel ratio(air_fuel_desired) and first output of exhaust sensor 140 (SIGNAL1).Next, in step 1226, fuel injection signal (fpw) is calculated in amanner similar to step 1114.

[0058] When the answers to either step 1218 or step 1220 are NO, theroutine continues to step 1228 to calculate fuel injection signal (fpw)as described herein in step 1116. Thus, according to the presentinvention, it is possible to utilize the first output of exhaust sensor140 for feedback-air-fuel control.

[0059] Referring now to FIG. 13, a routine is described for determiningdegradation of the second output signal of exhaust sensor 140. Inparticular, a routine is described for determining degradation ofindicated NOx concentration based on the first output signal of exhaustgas sensor 140, when the first output signal is indicative of an exhaustair-fuel ratio. First, in step 1310, a determination is made as towhether monitoring is enabled as described in step 1222, or whether theengine is operating in a near stoichiometric condition. Further, adetermination is also made as to whether the first output signal ofexhaust sensor 140 is degraded. In other words, when SIGNAL1 isindicative of the air-fuel ratio entering trap 72, it can be used toprovide an estimate of NOx concentration exiting trap 72. When theanswer to step 1310 is YES, the routine continues to step 1312. In step1312, the routine estimates the second output signal (est_signal 2)based on several conditions. In particular, function h2 is used with thefeed gas NOx (NOx_fg) and the first output signal of exhaust sensor 140(SIGNAL1). In other words, the routine attempts to estimate NOx exitingtrap 72 based on NOx entering trap 72 and exhaust air-fuel ratio. Inaddition, various other dynamic effects of NOx trap 72 can be added toaccount for oxygen storage and nitrogen oxide and oxygen reduction.Further, efficiency of trap 72 can be included to estimate NOx exitingbased on NOx entering trap 72. However, if performed duringstoichiometric operation, it can be assumed that net NOx stored isconstant. Next, in step 1314, the absolute value of the differencebetween the estimated NOx exiting trap 72 (EST_SIGNAL2) and measuredsecond output of exhaust sensor 140 (SIGNAL2) is compared to thresholdvalue C4. When the answer to step 1314 is YES, counter C5 is incrementedin step 1316. Next, in step 1318, a determination is made as to whethercounter C5 is greater than threshold C6. When the answer to step 1318 isYES, the routine indicates degradation of the second output of exhaustsensor 140 in step 1320.

[0060] Thus, according to the present invention, it is possible todetermine when the NOx sensor, which is the second output of exhaustsensor 140, has degraded by comparing to an estimated value of exitingNOx trap 72.

[0061] Referring now to FIG. 14, a routine is described for determiningdegradation of the second output signal of sensor 140 based on the firstoutput signal of sensor 140. First, a determination is made in step 1410as to whether monitoring has been enabled or whether operating nearstoichiometry. When the answer to step 1410 is YES, the routinecontinues to step 1412. In step 1412, the routine estimates air-fuelratio that should be measured by the first output signal (SIGNAL1) ofexhaust sensor 140. In other words, the routine estimates exhaustair-fuel ratio exiting NOx trap 72 based on various operatingparameters. The estimated air-fuel ratio (AFTP_EST) is estimated basedon air-fuel ratio measured by sensor 76 (UEGO), mass airflow measured bymass airflow sensor 100, and fuel injection amount (fpw) in a preferredembodiment. Those skilled in the art will recognize, in view of thisdisclosure, various other signals and methods that can be used toestimate exhaust air-fuel ratio exiting a NOx trap. For example, dynamiceffects of both catalyst 70 and 72 can be included that account for NOxstorage, oxygen-storage, temperature effects, and various other effectsknown to those skilled in the art.

[0062] Continuing with FIG. 14, in step 1414, the absolute value of thedifference between the estimated exhaust air-fuel ratio (AFTP_EST) inthe first output signal of exhaust gas sensor 140 (SIGNAL1) is comparedto threshold C7. When the answer to step 1414 is YES, counter C8 isincremented in step 1416. Next, in step 1418, counter C8 is compared tothreshold C9 in step 1418. When the answer to step 1418 is YES, anindication is provided in step 1420 that both the first output signaland a second output signal of exhaust sensor 140 have been degraded.Thus, according to the present invention, it is possible to determinethat the NOx concentration measured by the second output signal ofexhaust sensor 140 is degraded when it is determined that the oxygenpartial pressure indicated in the first output signal of exhaust sensor140 has been degraded.

[0063] Referring now to FIGS. 15A-15C, these figures show an example ofoperation according to the present invention. In particular, the graphsshow when first output signal (SIGNAL1) of sensor 140 is valid forair-fuel control or for monitoring. FIG. 15A shows air-fuel ratioentering NOx trap 72 versus time. FIG. 15B shows air-fuel ratio exitingNOx trap 72 versus time. FIG. 15C indicates whether first output signal(SIGNAL1) of sensor 140 is valid for air-fuel control or for monitoring.

[0064] Before time t1, the entering air-fuel ratio and exiting air-fuelratio are both lean and first output signal (SIGNAL1) is valid forcontrol or monitoring. Then, at time t1, a determination is made to endlean operation and purge NOx stored in trap 72 due to tailpipe grams ofNOx/mile, or because a fuel economy benefit is no longer provided byoperating lean, or for various other reasons as described above herein.At time t1, entering air-fuel ratio is changed from lean to rich.Similarly, at time t1, air-fuel ratio exiting changes to stoichiometricuntil all stored NOx and oxygen are reduced, which occurs at time t2.Thus, according to the present invention, the stoichiometric air-fuelratio measured downstream of NOx trap 72 during the interval from timet1 to time t2, is not equal to the air-fuel ratio upstream of NOx trap72. After time t2, a rich exhaust air-fuel ratio is measured downstreamof NOx trap 72 and this measurement can be used for air-fuel control ormonitoring. At time t3, entering air-fuel is changed back to a leanair-fuel ratio. Again, air-fuel ratio exiting changes to stoichiometricuntil all the oxygen storage capacity of NOx trap 72 is saturated attime t4. Thus, according to the present invention, the stoichiometricair-fuel ratio measured downstream of NOx trap 72 during the intervalfrom time t3 to time t4 is not equal to the air-fuel ratio upstream ofNOx trap 72. After time t4, the entering air-fuel ratio can be measuredby sensor 140 and thus can be used for control or monitoring.

[0065] Referring to FIG. 16, after the controller 12 has confirmed atstep 1610 that the lean-burn features is not disabled and, at step 1612,that lean-burn operation has otherwise been requested, the controller 12conditions enablement of the lean-burn feature, upon determining thattailpipe NOx emissions as detected by the NOx sensor 140 do not exceedpermissible emissions levels. Specifically, after the controller 12confirms that a purge event has not just commenced (at step 1614), forexample, by checking the current value of a suitable flag PRG_START_FLGstored in KAM, the controller 12 determines an accumulated measureTP_NOX_TOT representing the total tailpipe NOx emissions (in grams)since the start of the immediately-prior NOx purge or desulfurizationevent, based upon the second output signal SIGNAL2 generated by the NOxsensor 140 and determined air mass value AM (at steps 1616 and 1618).Because, in the exemplary system, both the current tailpipe emissionsand the permissible emissions level are expressed in units of grams pervehicle-mile-traveled to thereby provide a more realistic measure of theemissions performance of the vehicle, in step 1620, the controller 12also determines a measure DIST_EFF_CUR representing the effectivecumulative distance “currently” traveled by the vehicle, that is,traveled by the vehicle since the controller 12 last initiated a NOxpurge event.

[0066] While the current effective-distance-traveled measureDIST_EFF_CUR is determined in any suitable manner, in the exemplarysystem, the controller 12 generates the currenteffective-distance-traveled measure DIST_EFF_CUR at step 1620 byaccumulating detected or determined values for instantaneous vehiclespeed VS, as may itself be derived, for example, from engine speed N andselected-transmission-gear information. Further, in the exemplarysystem, the controller 12 “clips” the detected or determined vehiclespeed at a minimum velocity VS_MIN, for example typically ranging fromperhaps about 0.2 mph to about 0.3 mph (about 0.3 km/hr to about 0.5km/hr), in order to include the corresponding “effective” distancetraveled, for purposes of emissions, when the vehicle is traveling belowthat speed, or is at a stop. Most preferably, the minimum predeterminedvehicle speed VS_MIN is characterized by a level of NOx emissions thatis at least as great as the levels of NOx emissions generated by theengine 10 when idling at stoichiometry.

[0067] At step 1622, the controller 12 determines a modified emissionsmeasure NOX_CUR as the total emissions measure TP_NOX_TOT divided by theeffective-distance-traveled measure DIST_EFF_CUR. As noted above, themodified emissions measure NOX_CUR is favorably expressed in units of“grams per mile.”

[0068] Because certain characteristics of current vehicle activityimpact vehicle emissions, for example, generating increased levels ofexhaust gas constituents upon experiencing an increase in either thefrequency and/or the magnitude of changes in engine output, thecontroller 12 determines a measure ACTIVITY representing a current levelof vehicle-activity (at step 1624 of FIG. 16) and modifies apredetermined maximum emissions threshold NOX_MAX_STD (at step 1626)based on the determined activity measure to thereby obtain avehicle-activity-modified NOx-per-mile threshold NOX_MAX which seeks toaccommodate the impact of such vehicle activity.

[0069] While the vehicle activity measure ACTIVITY is determined at step1624 in any suitable manner based upon one or more measures of engine orvehicle output, including but not limited to a determined desired power,vehicle speed VS, engine speed N, engine torque, wheel torque, or wheelpower, in the exemplary system, the controller 12 generates the vehicleactivity measure ACTIVITY based upon a determination of instantaneousabsolute engine power Pe, as follows:

Pe=TQ*N*k _(I)

[0070] Where TQ represents a detected or determined value for theengine's absolute torque output, N represents engine speed, and k_(I) isa predetermined constant representing the system's moment of inertia.The controller 12 filters the determined values Pe over time, forexample, using a high pass filter G₁(s), where s is the Laplace operatorknown to those skilled in the art, to produce a high-pass filteredengine power value Hpe. After taking the absolute value AHPe of thehigh-pass-filtered engine power value Hpe, the resulting absolute valueAHPe is low-pass-filtered with filter G₁(s) to obtain the desiredvehicle activity measure ACTIVITY.

[0071] Similarly, while the current permissible emissions lend NOX_MAXis modified in any suitable manner to reflect current vehicle activity,in the exemplary system, at step 1626, the controller 12 determines acurrent permissible emissions level NOX_MAX as a predetermined functionf₅ of the predetermined maximum emissions threshold NOX_MAX_STD based onthe determined vehicle activity measure ACTIVITY. By the way of exampleonly, in the exemplary system, the current permissible emissions levelNOX_MAX typically varies between a minimum of about 20 percent of thepredetermined maximum emissions threshold NOX_MAX_STD forrelatively-high vehicle activity levels (e.g., for many transients) to amaximum of about seventy percent of the predetermined maximum emissionsthreshold NOX_MAX_STD (the latter value providing a “safety factor”ensuring that actual vehicle emissions do not exceed the proscribedgovernment standard NOX_MAX_STD.). See also FIG. 8.

[0072] Referring again to FIG. 16, at step 1628, the controller 14determines whether the modified emissions measure NOX_CUR as determinedin step 1622 exceeds the maximum emissions level NOX_MAX as determinedin step 1626. If the modified emissions measures NOX_CUR does not exceedthe current maximum emissions level NOX_MAX, the controller 12 remainsfree to select a lean engine operating condition in accordance with theexemplary system's lean-burn feature. If the modified emissions measureNOX_CUR exceeds the current maximum emissions level NOX_MAX, thecontroller 12 determines that the “fill” portion of a “complete”lean-burn fill/purge cycle has been completed, and the controllerimmediately initiates a purge event at step 1630 by setting suitablepurge event flags PRG_FLG and PRG_START_FLG to logical one.

[0073] If, at step 1614 of FIG. 16, the controller 12 determines that apurge event has been commenced, as by checking the current value for thepurge-start flag PRG_START_FLG, the controller 12 resets the previouslydetermined values TP_NOX_TOT and DIST_EFF_CUR for the total tailpipe NOxand the effective distance traveled and the determined modifiedemissions measure NOX_CUR, along with other stored values FG_NOX_TOT andFG_NOX_MOD (to be discussed below), to zero at step 1632. Thepurge-start flag PRG_START_FLG is similarly reset to logic zero at thattime.

[0074] Those skilled in the art will recognize in view of thisdisclosure that the above methods are applicable with anydecontamination method. In a preferred embodiment, the decontaminationmethod described in U.S. Pat. No. 5,758,493, which is herebyincorporated by reference, can be used.

[0075] Although several examples of embodiments which practice theinvention have been described herein, there are numerous other exampleswhich could also be described. The invention is therefore to be definedonly in accordance with the following claims.

1-17. (cancelled)
 18. An emission control system for an internalcombustion engine, comprising: a NOx absorbent disposed in an exhaustpassage of the internal combustion engine that stores and reacts NOxunder certain operating conditions; a NOx sensor disposed in the exhaustpassage downstream of the NOx absorbent, a first output of the NOxsensor corresponding to a NOx concentration of exhaust gas flowing outof the NOx absorbent and a second output of the NOx sensor correspondingto a oxygen concentration of exhaust gas flowing out of the NOxabsorbent; a controller calculating an operating condition of theinternal combustion engine and determining a deviation of the outputvalue of the NOx sensor from a predetermined value when preselectedengine operating conditions are met; and said controller furtherindicating whether predetermined engine operating conditions arepresent, and in response to said determination, adjusting a fuelinjection amount into the internal combustion engine based on saidsecond output.
 19. The system recited in claim 18 further comprising athree-way catalyst disposed in said engine exhaust passage upstream ofthe NOx absorbent.
 20. The system recited in claim 18 further comprisingan air-fuel ratio sensor disposed in the exhaust passage of the engineupstream of the NOx absorbent.
 21. The method recited in claim 18wherein said controller further changes engine operation from a leanair-fuel ratio to a stoichiometric or rich air-fuel ratio based on saidoutput of the NOx sensor.
 22. An emission control system for an internalcombustion engine, comprising: a NOx absorbent disposed in an exhaustpassage of the internal combustion engine that stores and reacts NOxunder certain operating conditions; a NOx sensor disposed in the exhaustpassage downstream of the NOx absorbent, an output of the NOx sensorcorresponding to a NOx concentration of exhaust gas flowing out of theNOx absorbent; a controller calculating an operating condition of theinternal combustion engine and determining a deviation of the outputvalue of the NOx sensor from a predetermined value when preselectedengine operating conditions are met; an air-fuel ratio sensor disposedin the engine exhaust passage; and said controller further indicatingwhether predetermined engine operating conditions are present, and inresponse to said determination, adjusting a fuel injection amount intothe internal combustion engine based on said air-fuel ratio sensor,wherein said controller further adjusts said fuel injection amount intothe internal combustion engine independent of said air-fuel ratio sensorwhen said controller indicates said predetermined engine operatingconditions are not present.
 23. An emission control system for aninternal combustion engine, comprising: a NOx absorbent disposed in anexhaust passage of the internal combustion engine that stores and reactsNOx under certain operating conditions; a NOx sensor disposed in theexhaust passage downstream of the NOx absorbent, an output of the NOxsensor corresponding to a NOx concentration of exhaust gas flowing outof the NOx absorbent; a controller calculating an operating condition ofthe internal combustion engine and determining a deviation of the outputvalue of the NOx sensor from a predetermined value when preselectedengine operating conditions are met; and said controller furtherperforming a sulfur decontamination process based on engine operatingconditions.